Hi thuway,
Although THX Ultra and Select certification contains some aspects of dubious benefit or minimal usefulness, the THX requirements for the amplifier output sections are quite rigorous and include the ability to drive 4-ohm loads without current limiting or amplifier shut-down from protection circuitry.
So look at receivers with THX certification. By the way, any receiver that will drive 8-ohm loads will have no trouble with 6-ohm loads. It's 4-ohm loads that cause problems with some brands that are basically spec'd for 8-ohm speakers and were never intended to be married to 4-ohm speakers.
Regards,
